1. Field of the Invention
The invention relates to modified oligonucleotides that are useful for studies of gene expression and for the antisense therapeutic approach.
2. Summary of the Related Art
The potential for using oligonucleotides as inhibitors of specific gene expression in an antisense therapeutic approach was first suggested in three articles published in 1977 and 1978. Paterson et al., Proc. Natl. Acad. Sci. USA 74: 4370-4374 (1977) discloses that cell-free translation of mRNA can be inhibited by binding a complementary oligonucleotide to the mRNA. Zamecnik and Stephenson, Proc. Natl. Acad. Sci. USA 75: 280-284 and 285-288 (1978) disclose that a 13-mer synthetic oligonucleotide that is complementary to a part of the Rous sarcoma virus (RSV) genome can inhibit RSV replication in infected cell cultures and can inhibit RSV-mediated transformation of primary chick fibroblasts into malignant sarcoma cells.
Since these early studies, the ability of antisense oligonucleotides to inhibit virus propagation has become firmly established. U.S. Pat. No. 4,806,463 teaches that human immunodeficiency virus propagation can be inhibited by oligonucleotides that are complementary to any of various regions of the HIV genome. U.S. Pat. No. 5,194,428 discloses inhibition of influenza virus replication by phosphorothioate oligonucleotides complementary to the influenza virus polymerase 1 gene. Agrawal, Trends in Biotechnology 10: 152-158 (1992) reviews the use of antisense oligonucleotides as antiviral agents.
Antisense oligonucleotides have also been developed as anti-parasitic agents. PCT publication no. WO93/13740 discloses the use of antisense oligonucleotides to inhibit propagation of drug-resistant malarial parasites. Tao et al., Antisense Research and Development 5: 123-129 (1995) teaches inhibition of propagation of a schistosome parasite by antisense oligonucleotides.
More recently, antisense oligonucleotides have shown promise as candidates for therapeutic applications for diseases resulting from expression of cellular genes. PCT publication no. WO95/09236 discloses reversal of beta amyloid-induced neuronal cell line morphological abnormalities by oligonucleotides that inhibit beta amyloid expression. PCT publication no. WO94/26887 discloses reversal of aberrant splicing of a globin gene transcript by oligonucleotides complementary to certain portions of that transcript. PCT application no. PCT/US94/13685 discloses inhibition of tumorigenicity by oligonucleotides complementary to the gene encoding DNA methyltransferase.
The development of various antisense oligonucleotides as therapeutic and diagnostic agents has recently been reviewed by Agrawal and Iyer, Current Opinion in Biotechnology 6: 12-19 (1995).
As interest in the antisense therapeutic approach has grown, various efforts have been made to improve the pharmacologic properties of oligonucleotides by modifying the sugar-phosphate backbone. U.S. Pat. No. 5,149,797 describes chimeric oligonucleotides having a phosphorothioate core region interposed between methylphosphonate or phosphoramidate flanking regions. PCT publication no. WO94/02498 discloses hybrid oligonucleotides having regions of 2'-O-substituted ribonucleotides flanking a DNA core region.
Much is currently being discovered about the pharmacodynamic properties of oligonucleotides. Agrawal et al., Clinical Pharmacokinetics 28: 7-16 (1995) and Zhang et al., Clinical Pharmacology and Therapeutics 58: 44-53 (1995) disclose pharmacokinetics of anti-HIV oligonucleotides in human patients. Some of these new discoveries have led to new challenges to be overcome for the optimization of oligonucleotides as therapeutic agents. For example, Kniep et al., Nature 374: 546-549 (1995) discloses that oligonucleotides containing the CG dinucleotide flanked by certain other sequences have a mitogenic effect. We have discovered that many side effects produced by phosphorothioate oligonucleotides are a consequence of the phosphorothioate-linked CpG dinucleotide. There is, therefore, a need for modified oligonucleotides that retain gene expression inhibition properties while producing fewer side effects than conventional phosphorothioate oligonucleotides.